Dark matter may not have been ‘cold’ in the earliest moments after the Big Bang, as long believed; instead, new research from the University of Minnesota Twin Cities and the Universit’e Paris-Saclay suggests dark matter particles could have been incredibly hot, traveling near the speed of light in the primordial cosmos, before cooling down in time to seed the formation of galaxies and large-scale structure.
Hypothetical dark matter particles. Image credit: University of Adelaide.
For decades, physicists have classified dark matter according to how fast its constituent particles moved, with cold dark matter being slow enough to clump together under gravity and help shape galaxies and galaxy clusters.
This model has been central to the standard cosmological framework, explaining the web-like structure of the Universe.
But the new findings suggest that dark matter could have decoupled from the early Universe’s hot plasma while still ultrarelativistic — essentially at extremely high speeds — and then cooled sufficiently before cosmic structures formed.
This nuanced picture expands the range of possible behaviors for dark matter particles, and widens the spectrum of candidate particles that physicists might pursue in experiments and astronomical observations.
The study hinges on a period in the early cosmos known as reheating, which followed the explosive expansion of the Universe called inflation.
During the post-inflationary reheating, the energy driving expansion converted into a hot soup of particles and radiation.
The findings indicate that, under certain conditions, dark matter produced at that time could begin life at near-light speeds yet still conform to the large-scale Universe we see today.
If correct, they may have profound implications for ongoing efforts to detect dark matter — whether through particle colliders, underground detectors, or astrophysical observations.
They also prompt new theoretical questions about the fundamental properties of dark matter and its role in cosmic evolution.
“Dark matter is famously enigmatic,” said Stephen Henrich, a graduate student at the University of Minnesota.
“One of the few things we know about it is that it needs to be cold.”
“As a result, for the past four decades, most researchers have believed that dark matter must be cold when it is born in the primordial Universe.”
“Our recent results show that this is not the case; in fact, dark matter can be red hot when it is born but still have time to cool down before galaxies begin to form.”
“The simplest dark matter candidate — a low mass neutrino — was ruled out over 40 years ago since it would have wiped out galactic size structures instead of seeding it,” said University of Minnesota’s Professor Keith Olive.
“The neutrino became the prime example of hot dark matter, where structure formation relies on cold dark matter.”
“It is amazing that a similar candidate, if produced just as the hot Big Bang Universe was being created, could have cooled to the point where it would in fact act as cold dark matter.”
“With our new findings, we may be able to access a period in the history of the Universe very close to the Big Bang,” said Professor Yann Mambrini, a physicist at the Universit’e Paris-Saclay.
The team’s work appears in the journal Physical Review Letters.
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Stephen E. Henrich et al. 2025. Ultrarelativistic Freeze-Out: A Bridge from WIMPs to FIMPs. Phys. Rev. Lett 135, 221002; doi: 10.1103/zk9k-nbpj